Re: IS-IS Flat Network Size

[Mark Tinka via NANOG <nanog () lists nanog org>]

On 30/07/2026 08:45, Matthew Petach via NANOG wrote:

> The point I was trying to make is that just because a routing protocol
> supports doing that doesn't mean it's a good idea operationally.  There
> are other issues that come into network design that often suggest a single
> flat IGP might not give you enough knobs to allow you to control traffic
> flows and impact radius from unusual externalities.

An obvious case against multi-level IS-IS is that to create end-to-end 
MPLS LSP's, you need each node to have every other node in its LFIB.

If you run a tiered L1 and L2 IS-IS network, you necessarily need to 
leak L2 routes into L1 for that full LFIB visibility. That automatically 
negates the need to have separate IS-IS levels.

Might as well run the whole network in L2.

Mark.
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